The invention relates to a cardiac defibrillator, particularly an implantable one, of the kind set forth in claim 1 and a corresponding method.
Defibrillators of this kind are known from sources such as European Pat. application No. 0 515 059. Defibrillators are generally implanted in increasing numbers in the case of patients who repeatedly suffer from fibrillation which requires electrotherapeutic help. To provide assistance without the presence of a doctorxe2x80x94more specifically because in many cases the doctor would not be on hand sufficiently quicklyxe2x80x94nowadays defibrillators of that kind are already being implanted in relatively large numbers and are thus available to the patient at any time. To keep down the size of those units while still providing sufficient energy even for repeated defibrillation procedures, optimum utilization of energy during an individual defibrillation procedure is a particularly important consideration.
The previously known defibrillator admittedly includes a plurality of capacitors which can be switched in different configurations. A disadvantage in that respect however that no indications whatsoever in regard to the sequence and the times of switching over the capacitors for optimization of the energy demand in a defibrillator of that kind are known. In this connection attention is directed to the following literature which however also does not provide any more detailed indications in this respect. It represents a summary of the previous endeavors to provide information about the energy demand in connection with defibrillators:
1. Schudder J C, Stoeckle H, West J A, et al: Transthoracic ventricular defibrillation in the dog with truncated exponential stimuli. IEEE Trans Biomed Eng BME 1971; 18: 410-415
2. Hamzei A, Mouchavar G, Badelt St et al: Three-capacitor multistep waveform lowers defibrillation threshold. PACE 1999; 22(5, II): abstract #87
3. Irnich W: The fundamental law of electrostimulation and its application to defibrillation. PACE 1990; 13: 1433-1447
4. Irnich W: Optimal truncation of defibrillation pulses. PACE 1995; 18: 673-688
5. Natale A, Sra J, Krum D et al: Relative efficacy of different tilts with biphasic defibrillation in humans. PACE 1996; 19: 197-206
6. Hahn St J, Heil J E, Lin Y et al: Optimization of 90 xcexcF biphasic defibrillation waveform for ICDs using a theoretical model and central composite design of experiments. PACE
7. Schauerte P, Schxc3x6ndube F A, Grossmann M et al: Optimized pulse duration minimizes the effect of polarity reversal on defibrillation efficacy with biphasic shocks. PACE 1999; 22: 790-797
8. Cleland B G: A conceptual basis for defibrillation waveforms. PACE 1996; 19: 1186-1195
9. Kroll M W: A minimal model of the monophasic defibrillation pulse. PACE 1993; 16: 769-777.
These references will be referred to hereinafter by these numbers.
Even these publications do not give any indications in regard to the stated problem, as will also be apparent from the systematic presentation hereinafter of the problems involved and the concept of the present invention.
An object of the present invention is to provide a defibrillator of the above-indicated kind or a corresponding defibrillation method, in which there are provided automatic control means which optimize the defibrillation effect with a plurality of capacitors.
The object is attained by the features recited in claim 1.
The object of the invention is attained by realizing that the degree of efficiency xcex7 (also referred to as xe2x80x9cetaxe2x80x9d) of the defibrillator in the various discharging procedures must be optimized in each case in such a way that the overall effect is also an optimum. The term xcex7 is used in electrical engineering to mean the xe2x80x9cefficiencyxe2x80x9d which normally defines the ratio of useful to applied energy. That consideration is based on an xe2x80x9cinputxe2x80x9d- to xe2x80x9coutputxe2x80x9d-comparison which in the case of the defibrillator would have to be such that the energy taken from the battery is compared to the energy delivered to the heart. Here however the notion of efficiency is additionally expanded in such a way that it embraces more than a straightforward input-output calculation, but also includes the question of the biological effectiveness of different pulse shapes.
The problem involved can be illustrated by reference to two examples: A favorable input-output ratio would be achieved in defibrillation if the output capacitor or capacitors was or were completely discharged. The efficiency would be 1. However Schudder et al (Ref. 1) already found in 1970 that the effectiveness is increased if the capacitor or capacitors is or are not completely discharged, but the discharge procedure is prematurely interrupted (truncated or curtailed). Now, it is worth noting that the optimum xe2x80x9ctiltxe2x80x9d (this English expression to denote slope or gradient is an established part of defibrillator terminology, and it should better be referred to as xe2x80x9cdegree of utilizationxe2x80x9d) has not hitherto been systematically investigated. On the contrary, the electrophysiological problem was made more complicated by the fact it was postulated as being self-evident that the optimum xe2x80x9ctiltxe2x80x9d, once found, enjoyed general applicability. However the engineers of the defibrillator manufacturer which for a long time was the only one prejudiced the discussion about optimum tilt by virtue of the fact that they implemented the idea which from the point of view of electrical engineering appears a reasonable one of providing for discharge of the output capacitor or capacitors to 20% of the initial voltage (corresponding thereto is a degree of utilization or tilt of 80%), to which there corresponds an efficiency of 96% (as the residual voltage is involved in quadratic terms in the energy calculation). That assumption was not derived from any defibrillation experiments but was based on purely electrical engineering considerations.
As a second example mention may be made of a poster (Ref. 2) which was displayed in Toronto, Canada, in May 1999, on the occasion of the NASPE-Conference. The authors reported that, by virtue of serial connection of three output capacitors previously discharged to about 85%, they had required a lesser amount of stored energy in comparison to just one capacitor which was discharged to 45%. They explained that increased efficiency on the basis that a pulse with a rising pulse shape is more desirable, on the basis of the xe2x80x9cmembrane-response-modelxe2x80x9d hypothesis thereof. That interpretation cannot be reconciled with the basic law of electrostimulation which is also applicable in regard to defibrillation (Refs. 3, 4). The fact that nonetheless a first parallel and then a serial discharge can be advantageous involves reasons related to electrical engineering, which will be discussed in greater detail hereinafter.
According to the present invention, however, a superior defibrillator is provided compared to the known defibrillators. When ascertaining the tilt or the normalized residual voltage in the discharge procedure in the prior art, measurements were made directly on the patient and which thus best correspond to the prevailing factors by virtue of the fact that the residual voltage is adapted to the defibrillation impedance upon discharge or in terms of tilt.
In this respect consideration was given inter alia to the fact that stimulation and defibrillation obey the law which was already published in 1909 by Lapicque and which can be formulated as follows:
U(mean)=Urheobase(1+Tchronaxie/T)xe2x80x83xe2x80x83(1)
wherein:
U(mean)=mean voltage during a stimulation pulse,
Urheobase=the voltage which just still stimulates with an infinitely long pulse duration (a more theoretical value),
Tchronaxie=pulse duration at double the rheobase value.
The present invention is based upon the realization that two rules apply in regard to the effect of defibrillation as a function of the pulse duration:
the voltage-time integral which increases linearly with the pulse duration is decisive, and
the pulse worsens the defibrillation effect if it falls below a given value, the above-mentioned xe2x80x9crheobasexe2x80x9d value.
Accordingly, it is further concluded therefrom that two pulses of different shape achieve the same effect if the mean value of their voltage is equal and none of the pulses has components below the rheobase. It can be mathematically deduced that, in an exponential procedure, the mean value can be correspondingly calculated as follows:
U(mean)=(U(o)xe2x88x92U(residue)):In(U(o)/U(residue))xe2x80x83xe2x80x83(2)
wherein:
U(o)=initial voltage to which the capacitor was charged,
U(residue)=residual voltage at the end of the pulse which in accordance with the theory is identical to the rheobase value, that is to say
U(residue)=Urheobase 
If the mean voltage is related to the initial value U(o), that affords a normalized mean voltage (NMV):
NMV=U(mean)/U(o)=(1-U(residue)/U(o)):In(U(o)/U(residue))xe2x80x83xe2x80x83(3)
The above-mentioned tilt is also determined from the values U(o) and U(residue), more specifically in accordance with equation (4):
tilt=1-U(residue)/U(o)xe2x80x83xe2x80x83(4)
from which it is possible to deduce the following:
U(residue)/U(o)=1-tiltxe2x80x83xe2x80x83(5)
and
U(o)/U(residue)=1:(1-tilt)xe2x80x83xe2x80x83(6)
Equation (3) can then be correspondingly written as follows:
NMV=tilt:In(1:(1-tilt))xe2x80x83xe2x80x83(7)
For all exponential discharge, that means that the mean value is always equal if only U(o) and U(residue) or tilt are equal.
Assuming that two partial capacitors are discharged first in parallel and then, after they have been discharged to a residual voltage U(residue), they are discharged by serial connection from double the residual voltage again to U(residue), the overall duration of the discharge process is equal to that of a single capacitor with the same initial and residual voltages U(o) and U(residue), and the following can be formulated:
U(residue)=U(o)exp(-t/RC), there from: t=RC *1n (U(o):U(residue))xe2x80x83xe2x80x83(8).
For any ratio of U(residue):U(o), we obtain from a comparison of the time of the individual capacitor C1 to the total time of the two partial capacitors C2:
RC1 1n(U(o):U(residue))=RC2 1n(U(o):U(residue))+RC2 1n (U(o):U(residue))+xc2xdRC2 1n 2xe2x80x83xe2x80x83(9)
Or, expressed with equation (6):
RC1 1n(1:(1-tilt))=RC2 {2 1n(1:(1-tilt))+0.5 1n2}xe2x80x83xe2x80x83(10)
with 1n 2 in the terms at the right in (9) and (10) as the doubled residual voltage is again discharged to the residual voltage at half the capacitance.
A conditional equation for RC2 or for the ratio C2/C1 can be derived from equation (10), as follows:
RC2 =RC1 1n(1:(1-tilt)):{2 1n(1:(1-tilt))+0.5 1n 2}xe2x80x83xe2x80x83(11a) and
C2/C1 =1n (1:(1-tilt)):{2 1n(1:(1-tilt))+0.5 1n2}xe2x80x83xe2x80x83(11b)
In regard to the discharge of a single capacitor, it was deduced from the theory of defibrillation (Ref. 4) that a respective optimum tilt is associated with the capacitor. That makes the seemingly complicated replacement of U(o) and U(residue) by tiltxe2x80x94also in connection with the invention described hereinxe2x80x94understandable; for, it can be directly looked up so that evaluation can be affected not only by calculation but directly also with a look-up table.
If it is formulated that the capacitors are to discharge to half the voltage, and the tilt is then 50%, equation (11a) simply gives as follows:
C2=C1 1n (1:(1-50%)):{2 1n (1:(1-50%))+0.5 1n2}=C1xc2x71:2.5=0.4C1xe2x80x83xe2x80x83(12)
As for the individual discharge processes of RC2 the same respective mean value is afforded in accordance with equation (2) and (7) respectively and as moreover the duration for both forms of discharge is identical in accordance with equation (9), both must also have the same defibrillation effect. What is of particular significance is the fact that the stored energy for both forms of discharge is different, with the same initial voltage:
E1=0.5xc2x7C1xc2x7U(o)2 and E2=0.5xc2x70.8xc2x7C1xc2x7U(o)2xe2x80x83xe2x80x83(13)
and:
E2:E1=0.8xe2x80x83xe2x80x83(14)
Therefore the parallel/serial discharge needs 20% less energy to arrive at the same result. In that respect, it is immaterial whether the two capacitances are firstly discharged simultaneously parallel or sequentially, before they are then discharged serially.
All in all it is possible in that way with the features according to the invention for the moments in time and/or the sequence of switching over between different capacitor configurations in the discharge procedure to be ascertained in the optimum manner and for the appropriate switching operations to be correspondingly triggered.
According to the invention, therefore, a defibrillator for atrium and/or ventricle is provided with at least two output capacitors which upon defibrillation are discharged in at least two phases in succession in different configurations, wherein the discharge in the at least two discharge phases is controlled by suitable switching means in such a way that the mean value of their voltages is substantially equal and voltages below the rheobase do not occur in any of the discharge phases.
In this way it is possible also to achieve optimum defibrillation results in systems with multi-capacitor arrangements, while an additional degree of freedom in terms of dimensioning of defibrillators is achieved in that the most widely varying types of capacitors and arrangements can be used, as are appropriate for reasons of space or other optimization reasons and nonetheless optimum utilization of the available energy is made possible for those capacitors. In that fashion the defibrillators can provide their service for a very long period of time without re-implantation.
If there are provided further switching means which break off the discharge procedure in the first discharge phase in dependence on the ascertained discharge time constant xe2x80x94which is determined by the capacitor and the electrode resistancexe2x80x94upon the attainment of a predetermined tilt or the corresponding residual voltage or a discharge time duration which is to be expected and which is previously calculated on the basis of the ascertained time constant until attainment of the tilt or the corresponding residual voltage and which correspondingly continue the discharge in a second phase with a series connection of the capacitors with voltage doubling, the above-mentioned aim can be embodied with a circuit arrangement which is simple to carry into effect or with a corresponding software-controlled system.
If the first configuration involves switching means which control the discharge of a single capacitor, the discharge of two parallel-connected capacitors or the sequential discharge of two individual capacitors, the invention can be advantageously used in relation to various design configurations of defibrillators.
It is particularly advantageous if there are provided switching means which determine the discharge time constant in the first phase in the first configuration during the discharge so that it is possible in each case to have recourse to the value ascertained in that way in respect of the normalized residual voltage or tilt in subsequent discharge procedures for other capacitor configurations.
Desirably it is also possible to provide switching means which with predetermined impedance values, instead of a single-capacitor or multi-capacitor system, also activate the individual discharge of a capacitor (one-capacitor system) if that is desirable from the point of view of energy in particular cases.
As the residual voltage remaining on the capacitor after defibrillation in accordance with the above-mentioned principles generally cannot be xe2x80x9cheldxe2x80x9d until a later defibrillation procedure occurs, it seems appropriate if there are provided switching means for subsequently discharging that residual voltage in inverted mode (as a bi-phase system). That is preferably effected in a parallel circuit configuration.
To permit individual adaptation to individual factors of the patient concerned in individual cases, it is advantageous if the ascertained tilt or the corresponding residual voltage is additionally variable by substantially plus/minus 20% as a function of the time constant RC1 or RC2.
Similar considerations apply in regard to the corresponding method claims.